In the aforementioned prior arts, it has been inevitable that the recovery of product nitrogen is decreased several percent to more than ten percent (proportionally to the amount of product oxygen) because nitrogen feed gas rising in a nitrogen rectification column is decreased in amount.
Due to consideration of the aforementioned problems, the present invention is intended to provide a unit which can produce a small amount of oxygen at the same time, without decreasing the amount of nitrogen produced, by solving these drawbacks.